The use of inorganic corrosion inhibitors, e.g. metal oxides alone and/or in combination with organic inhibitors including phosphonic acids have been used in various aqueous systems. It has been found, in accordance with this invention, the hexamethylenediamine tetra(methylene phosphonic acid), the dimer of hexamethylenediamine tetra(methylene phosphonic acid) and the various derivatives thereof when used in combination with a certan amount of molybdate have improved corrosion inhibition and prevent the deposition of mineral scale.
In general, corrosion is defined as a destructive attack on metal involving an electrochemical or chemical reaction of the metal with its environment. More specifically, an electrochemical attack on a metal surface is the wearing away and undercutting of the metal, which is accelerated after the protective coating, e.g. the oxide film is removed by the corrosive medium. Other types of corrosion include cavitation and erosion where, in addition to an electrochemical reaction, the condition of the aqueous system is such that the continuous flow causes cavities where high pressure areas develop causing pressure and shock resulting in a pitted metal surface. This type of corrosion generally is found in water pumps, propellers, turbine blades, etc. Further, erosion of the metal surface occurs if the medium contains suspended solids which impinge on the surface of the metal as the fluid is transported thereby removing any protective film and exposing the metal to corrosion.
Presently, because of the environmental problems, there is a need to use many of the known corrosion inhibiting compositions at lower concentrations. Often these compositions contain an agent to control scale formation which, however, has a tendency to increase the rate of corrosion. Therefore, it is becoming necessary to use stronger corrosion inhibitors at higher concentrations in order to obtain satisfactory results. The use of some of the inhibitors, e.g. the chromates, etc. at the higher concentrations is unsatisfactory due to the environmental restrictions. It has been found, however, that by utilizing an aminomethylene phosphonic acid or its derivative, i.e. hexamethylenediamine tetra(methylene phosphonic acid) with a molybdate, lower concentrations of these undesirable inhibitors can be used in combination therewith and in many instances even a weaker inhibitor will provide satisfactory results. The compositions of this invention not only eliminate the need for using toxic materials at the higher concentrations, but also provides a corrosion inhibitor effective in a number of different systems.